February 2, 2012 — Rice University studied the fluorescence of single-walled carbon nanotubes (SWCNT) in new research, finding that the lengths and imperfections of individual nanotubes affect their fluorescence. This research focused on the light SWCNTs emit at near-infrared wavelengths.
The brightest nanotubes of a given length show consistent fluorescence intensity. The longer the nanotube, the brighter it fluoresces. "Maximum brightness is proportional to length," noted Bruce Weisman, who led the research. Weisman found a "well-defined limit" to the SWCNTs brightness. Brightness among nanotubes of the same length varied widely, likely due to damaged or defective structures or chemical reactions that allowed atoms to latch onto the surface.
Figure. Carbon nanotubes of varying fluorescence in a solution at Rice University.
Former graduate student Tonya Leeuw Cherukuri analyzed 400 individual nanotubes of a specific physical structure known as (10,2). The researchers applied spectral filtering to selectively view the specific type of nanotube. "We used spectroscopy to take this very polydisperse sample containing many different structures and study just one of them, the (10,2) nanotubes," Weisman said. "But even within that one type, there’s a wide range of lengths."
Weisman, Cherukuri, and postdoctoral fellow Dmitri Tsyboulski isolated 1 or 2 nanotubes at a time in a dilute sample, finding their lengths by analyzing videos of the moving tubes captured with a special fluorescence microscope. The movies also allowed Cherukuri to catalog their maximum brightness.
The researchers called these CNTs "fluorescence underachievers," because only a few bright ones fluoresce to their full potential. Most are only 50-20% of their potential brightness.
By studying the nanotubes, Rice University hopes to discover how their fluorescence is affected by growth methods and processing. This way, the researchers could minimize damage during nanotube manufacturing, potentially lessening or eliminating the dimming.
"These are insights you really can’t get from measurements on bulk samples," Weisman said.
Weisman said brightness properties may be important to medical imaging and industrial applications.
Graduate student Jason Streit is extending Cherukuri’s research, developing a way to automate the experiments to image and analyze dozens of nanotubes at once.
The research was supported by the Welch Foundation, the National Science Foundation and Applied NanoFluorescence.
Bruce Weisman’s Rice U. lab published the results in the current issue of the American Chemical Society journal ACS Nano. Access the ACS Nano article "How Nanotubes Get Their Glow": http://pubs.acs.org/doi/abs/10.1021/nn2043516
See a video of fluorescent carbon nanotubes moving in a solution at http://youtu.be/4ceWLcOMxz0. SOURCE: Jason Streit/Rice University.
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